Process steps for manufacturing a semiconductor device includes a silicon nitride film etching step that immerses substrates (e.g., semiconductor wafers) in a phosphoric acid solution stored in a processing bath so that silicon nitrides films formed on the surfaces of the substrates are subjected to wet etching.
During etching, silicon derived from the silicon nitride film dissolves in the phosphoric acid solution. For the purpose of increasing the etch selectivity of the silicon nitride film to the silicon oxide film (i.e., the ratio of the silicon nitride etch rate to the silicon nitride etch rate), and of reducing particles derived from the dissolved silicon, etc., it is necessary to maintain the dissolved silicon concentration in the phosphoric acid solution within a predetermined range.
In JP2015-070080A (Patent Document 1), when substrates are being immersed in a phosphoric acid solution in a processing bath, the phosphoric acid solution in the processing bath is continuously drained at a predetermined constant drainage flow rate, while a phosphoric acid solution free of silicon (or phosphoric acid of low silicon concentration) is supplied into the processing bath at a constant flow rate which is the same as the drainage flow rate, whereby the silicon concentration in the phosphoric acid solution in the processing bath gradually increases when the substrates are being immersed in the phosphoric acid solution in the processing bath.
JPH09-275091A (Patent Document 2) describes a substrate processing apparatus including a silicon injection apparatus that injects silicon, e.g., colloidal silicon into a circulation line connected to a processing bath. The apparatus is configured to control the silicon concentration in the phosphoric acid solution within a desired range by injecting an amount of silicon, with the use of the silicon injection apparatus, which is determined based on the silicon concentration in the phosphoric acid solution detected by a silicon concentration sensor. Patent Document 2 describes that, in addition to the above, the apparatus is further provided with a drain for draining the phosphoric acid solution, which is being used, from the circulation line, and a phosphorous acid solution adding tube for adding a new phosphoric acid solution to the circulation line. Due to the injection of silicon, the drainage of the phosphoric acid solution which is being used and the addition of the new phosphoric acid solution, the silicon concentration in the phosphoric acid solution can be maintained within a desired range. Patent Document 2 does not describe that the amount of silicon contained in the phosphoric acid solution is changed among two or more different target values during the processing of substrates of one process lot.
JP2001-023952A (Patent Document 3) describes that, when silicon concentration in a phosphoric acid solution in a processing bath, which is detected by a concentration detection sensor, reaches a predetermined threshold value, a part of the phosphoric acid solution in the processing bath is drained and a new phosphoric acid solution is added to the processing bath during the interval between processing of substrates of a certain process lot and processing of substrates of the next lot, so as to maintain the silicon concentration in the phosphoric acid solution in the processing bath within a predetermined range. Patent Document 3 does not describe that, during the processing of substrates of one process lot, the phosphoric acid solution which is being used is drained from the processing bath and a new phosphoric acid solution is added thereto.
The technique of Patent Documents 1 to 3 cannot perform an etching process at different controlled etching rates, during the processing of substrates of one process lot.